NASA - Top Story - ARE CITIES CHANGING LOCAL AND GLOBAL CLIMATES? - December 11, 2003

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In summer, weaker winds move the clouds more slowly. Heat absorbed by the city and pollution's interference with raindrop formation interact to cause the clouds to intensify before producing precipitation. The onset of rainfall from a cloud leads eventually to its demise by cooling off the air near the ground. The air pollution delays the onset of precipitation, so that the intense storm clouds can build higher and larger before they start precipitating and subsequently dissipating. Therefore, these larger and more intense thunderstorm clouds produce eventually heavier rainfall on the city and the downwind areas. First is the unpolluted, then the polluted case. Credit: NASA

Cities tend to be 1-10 degrees Fahrenheit warmer than surrounding areas. The added heat destabilizes and changes air circulation around cities. During the warmer months, the added heat creates wind circulations and rising air that produces new clouds or enhances existing ones. Under the right conditions, these clouds evolve into rain-producers or storms. Scientists suspect that converging air due to city surfaces of varying heights, like buildings, also promotes rising air needed to produce clouds and rainfall. Credit: NASA

In winter, moist air flows off the ocean and rises over the hills downwind of a coastal city, dropping its rain and snow mainly as it ascends the hills. As pollution from the city is pushed into the clouds by the hills downwind of the city, it interferes with droplet formation in the clouds and makes them smaller, as observed by NASA's satellites. The smaller cloud droplets convert more slowly into precipitation. Instead of precipitating, much of the water in the clouds evaporates, reducing the net rainfall downwind of the urban area by up to 15% to 25% on a seasonal basis. First is the unpolluted, then the polluted case. Credit: NASA

Normal rainfall droplet creation involves water vapor condensing on particles in clouds. The droplets eventually coalesce together to form drops large enough to fall to Earth. However, as more and more pollution particles (aerosols) enter a rain cloud, the same amount of water becomes spread out. These smaller water droplets float with the air and are prevented from coalescing and growing large enough for a raindrop. Thus, the cloud yields less rainfall over the course of its lifetime compared to a clean (non-polluted) cloud of the same size. The split screen compares a normal rain producing cloud (left) with the lack of rain produced from a cloud full of aerosols from pollution. Credit: NASA

Caption for Image 5:Satellite Images of Houston Metro Area

These images show the Houston metropolitan area, where buildings, roads and other built surfaces create urban heat islands that can affect local rain patterns. The images were taken by ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer), an imaging instrument that is flying on Terra, a satellite launched in December 1999 as part of NASA's Earth Observing System (EOS). Credit: NASA/JPL

The Tropical Rainfall Measuring Mission (TRMM) is the first Earth Science mission dedicated to studying tropical and subtropical rainfall, precipitation that falls within 35 degrees north and 35 degrees south of the equator. Tropical rainfall comprises more than two-thirds of the world's total. The satellite uses several instruments to detect rainfall including radar, microwave imaging, and lightning sensors. Flying at a low orbital altitude of 217 miles (350 kilometers), TRMM's study of tropical rainfall and attendant processes will help improve our understanding and predictions of global climate change. The Japanese space agency (NASDA) launched the satellite on an H-II rocket from Tanegashima Space Center on November 27, 1997. TRMM data is available to researchers around the world; it is managed by a team at NASA's Goddard Space Flight Center in Greenbelt, Maryland. Credit: NASA

Caption for Item 9:Terra Satellite

The five sensors aboard Terra are comprehensively measuring our world's climate system-to observe and measure how Earth's atmosphere, cryosphere, lands, oceans, and life all interact. Data from this mission are used in many research and commercial applications. Terra is a vital part of NASA's Earth Science Enterprise, helping us understand and protect our home planet. Credit: NASA

Caption for Item 10:Aqua Satellite

Aqua launched May 4, 2002, a powerful Earth observing platform. Aqua's six advanced instruments will look at interrelated geophysical properties of our home planet, with a particular emphasis on water. Comprehensive measurements taken by Aqua's onboard instruments will enable scientists to assess long-term climate change, identify its human and natural causes and advance the development of models for long-term forecasting. Credit: NASA

Caption for Item 11:AuraSatellite

Aura will supply the most complete information yet on the health of Earth's atmosphere, once it is launched in spring 2004. This satellite will help scientists understand the causes behind worsening global air quality, our rapidly changing climate, and track the predicted recovery of the ozone layer. Aura will collect data on the composition, chemistry and dynamics of the Earth's upper and lower atmosphere employing multiple instruments on a single satellite. Aura's measurements will follow up on records that began with NASA'S Upper Atmospheric Research Satellite (UARS) and the Total Ozone Mapping Spectrometer (TOMS). Credit: NASA

New evidence from satellites, models, and ground observations reveal urban areas, with all their asphalt, buildings, and aerosols, are impacting local and possibly global climate processes. This is according to some of the world's top scientists convening in a special session at the Fall Meeting of the American Geophysical Union in San Francisco.

Item 2 - Coastal City

To study urban impact on local rainfall, Dr. J. Marshall Shepherd of NASA's Goddard Space Flight Center, Greenbelt, Md., and Steve Burian of the University of Utah, Salt Lake City, used the world's first space-based rain radar, aboard the Tropical Rainfall Measuring Mission (TRMM) satellite, and dense rain gauge networks on land to determine there are higher rainfall rates during the summer months downwind of large cities like Houston and Atlanta. Burian and Shepherd offer new evidence that rainfall patterns and daily precipitation trends have changed in regions downwind of Houston from a period of pre-urban growth, 1940 to 1958, to a post-urban growth period, 1984 to 1999.

Item 3 - Winter

Cities tend to be one to 10 degrees Fahrenheit (0.56 to 5.6 degrees Celsius) warmer than surrounding suburbs and rural areas. Warming from urban heat islands, the varied heights of urban structures that alter winds, and interactions with sea breezes are believed to be the primary causes for the findings in a coastal city like Houston.

In related work, Dr. Daniel Rosenfeld, an atmospheric scientist at Hebrew University, Jerusalem, reveals the increased amount of aerosols, tiny air particles, added by human activity to those naturally occurring also alter local rainfall rates around cities. Rosenfeld suggests the particles provide many surfaces upon which water can collect, preventing droplets from condensing into larger drops and slowing conversion of cloud water into precipitation. In summer, rain and thunder increases downwind of big cities, as rising air from urban heat islands combines with 'delayed' rainfall resulting from the presence of aerosols, creating bigger clouds and heavier rain.

Item 4

To help scientists like Shepherd and Rosenfeld improve understanding of links between city landscapes and climate processes like rainfall, NASA's suite of Earth observing satellites provides information about the land cover/land use properties that initiate the urban effects. The satellites also track the aerosols, clouds, water vapor, and temperature that describe atmospheric conditions in urban environments. Their measurements allow scientists to make end-to-end studies of urban impacts on the climate system practically anywhere on Earth.

Item 5

"The space-borne instruments on Terra, Aqua, TRMM, and Landsat provide a wealth of new observations of aerosol particles near and downwind of cities, the cloud optical properties, and surface reflectance characteristics that can help us understand the effects that urban environments have on our atmosphere and precipitation patterns," said Dr. Michael King, NASA Earth Observing System Senior Project Scientist. "Aura, to be launched in 2004, will add even more data," he said.

With growing evidence of the effects of urbanization on climate, climate modelers, like Georgia Institute of Technology's Dr. Robert Dickinson, hope to account for the cumulative effects of urban areas on regional and global climate models. For example, since asphalt has such a large effect on local heat transfer, water run-off, and how winds behave, characterizing asphalt cover is probably the biggest urban effect to be factored into global models.

NASA's Earth Science Enterprise is dedicated to understanding the Earth as an integrated system and applying Earth System Science to improve prediction of climate, weather and natural hazards using the unique vantage point of space.

Shepherd, King, Rosenfeld, and Dickinson will present their findings during a press conference on Thursday, December 11, 2003, at 3 p.m., PST in Room 2012, Moscone West, at the 2003 Fall Meeting of the American Geophysical Union in San Francisco.

They also will convene a special session, organized by Shepherd and Dr. Menglin Jin of the University of Maryland, detailing these results on Human-Induced Climate Variations Linked to Urbanization: From Observations to Modeling, sessions U51A and U51C, starting on Friday morning, December 12, at 2:00 p.m. PST at MCC 3001-3003. B-roll of video is available on this topic, by calling Wade Sisler of NASA-TV at 301/286-6256.